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Intersystem Crossing in Diplatinum Complexes

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journal contribution
posted on 26.09.2016, 16:53 by Yan Choi Lam, Harry B. Gray, Jay R. Winkler
Intersystem crossing (ISC) in solid [(C4H9)4N]4[Pt2(μ-P2O5(BF2)2)4], abbreviated Pt­(pop-BF2), is remarkably slow for a third-row transition metal complex, ranging from τISC ≈ 0.9 ns at 310 K to τISC ≈ 29 ns below 100 K. A classical model based on Boltzmann population of one temperature-independent and two thermally activated pathways was previously employed to account for the ISC rate behavior. An alternative we prefer is to treat Pt­(pop-BF2) ISC quantum mechanically, using expressions for multiphonon radiationless transitions. Here we show that a two-channel model with physically plausible parameters can account for the observed ISC temperature dependence. In channel 1, 1A2u intersystem crosses directly into 3A2u using a high energy B–F or P–O vibration as accepting mode, resulting in a temperature-independent ISC rate. In channel 2, ISC occurs via a deactivating state of triplet character (which then rapidly decays to 3A2u), using Pt–Pt stretching (160 cm–1) as a distorting mode to provide the energy needed. Fitting indicates that the deactivating state, 3X, is moderately displaced (S = 0.5–3) and blue-shifted (ΔE = 1420–2550 cm–1) from 1A2u. Our model accounts for the experimental observation that ISC in both temperature independent and thermally activated channels is faster for Pt­(pop) than for Pt­(pop-BF2): in the temperature independent channel because O–H modes in the former more effectively accept than B–F modes in the latter, and in the thermally activated pathway because the energy gap to 3X is larger in the latter complex.